This is one of my favorite analogies from C.S. Lewis. This quote is from Mere Christianity:
I find a good many people have been bothered by what I said in the previous chapter about Our Lord’s words, `Be ye perfect.’ Some people seem to think this means ‘Unless you are perfect, I will not help you’; and as we cannot be perfect, then, if He meant that, our position is hopeless. But I do not think He did mean that. I think He meant ‘The only help I will give is help to become perfect. You may want something less: but I will give you nothing less.’
Let me explain. When I was a child I often had toothache, and I knew that if I went to my mother she would give me something which would deaden the pain for that night and let me get to sleep. But I did not go to my mother-at least, not till the pain became very bad. And the reason I did not go was this. I did not doubt she would give me the aspirin; but I knew she would also do something else. I knew she would take me to the dentist next morning. I could not get what I wanted out of her without getting something more, which I did not want. I wanted immediate relief from pain: but I could not get it without having my teeth set permanently right. And I knew those dentists; I knew they started fiddling about with all sorts of other teeth which had not yet begun to ache. They would not let sleeping dogs lie; if you gave them an inch they took an ell.
Now, if I may put it that way, Our Lord is like the dentists. If you give Him an inch, He will take an ell. Dozens of people go to Him to be cured of some one particular sin which they are ashamed of (like masturbation or physical cowardice) or which is obviously spoiling daily life (like bad temper or drunkenness). Well, He will cure it all right: but He will not stop there. That may be all you asked; but if once you call Him in, He will give you the full treatment.
That is why He warned people to ‘count the cost’ before becoming Christians. ‘Make no mistake; He says, ‘if you let me, I will make you perfect. The moment you put yourself in My hands, that is what you are in for. Nothing less, or other, than that. You have free will, and if you choose, you can push Me away. But if you do not push Me away, understand that I am going to see this job through. Whatever suffering it may cost you in your earthly life, whatever inconceivable purification it may cost you after death, whatever it costs Me, I will never rest, nor let you rest, until you are literally perfect-until my Father can say without reservation that He is well pleased with you, as He said He was well pleased with me. This I can do and will do. But I will not do anything less.’
You can read Mere Christianity online here. In case you are wondering, an “ell” is an English unit of measurement which equals 45 inches, i.e. 1.143 meters.